This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to JP2001-151831 filed in Japan on May 22, 2001; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a lightweight wave gear device that is equipped with a rigid internal gear made of a lightweight alloy.
2. Related Art
A wave gear device is usually composed of a ring-shaped rigid internal gear, a flexible external gear that is also ring-shaped and is disposed on an inside of the rigid internal gear, and a wave generator that is fitted into an inside of the flexible external gear. The wave generator is composed of a rigid wave plug with an oval outline and a wave bearing that is fitted on an outer circumferential surface of the wave plug, so that the external gear is bent into an oval shape and the external teeth positioned at both ends of the major axis of the flexible external gear engage the internal teeth of the rigid internal gear. When the wave generator is rotated by a motor or the like, the engaging parts of both gears move in a circumferential direction. As a result, the two gears rotate relative to one another due to a difference in the number of teeth on each gear. This difference in the number of teeth is normally two, and the rigid internal gear is normally fixed to a device housing or the like, so that a rotational output of a speed that is significantly reduced, based on the difference in the number of teeth, is obtained from the flexible external gear.
In order to reduce the weight of a wave gear device of the above construction, the inventors of the present invention have proposed manufacturing the rigid internal gear and wave generator from a lightweight alloy, such as an aluminum alloy. One example of such a lightweight wave gear device is disclosed in JP-A 10-318338.
Compared to the normal case where a rigid internal gear is manufactured from a ferrous material, a rigid internal gear that is made of aluminum alloy or another lightweight alloy suffers from a large decrease in the engagement rigidity (which is to say, there is a decrease in ratcheting torque), resulting in gear slippage. In order to achieve a similar level of rigidity as that achieved by a rigid internal gear made of a ferrous material, the external diameter and width of the rigid internal gear have to be increased. Such changes in the dimensions of the rigid internal gear make it necessary to change the dimensions of the part of a device, such as a robot, to which the wave gear device is attached. In other words, such changes make it necessary to change the user device specification for the wave gear device. Such changes to the dimensions are usually very difficult to achieve.
When the rigid internal gear is formed of a lightweight alloy, there is a further problem in that surface hardness and abrasion resistance of the gear teeth are lower than when a ferrous material is used. Furthermore, there is the risk of surface parts, which act as the bearing surfaces for fastening bolts that attach the rigid internal gear to other members, being unable to withstand the pressure that acts on the bearing surfaces and so becoming deformed.
The present invention was conceived in view of the above problems, and has an object of providing a wave gear device where the weight of the rigid internal gear can be reduced without changing the dimensions of the rigid internal gear.
A second object of the present invention is to provide a wave gear device where the weight of the rigid internal gear can be reduced without causing decreases in the abrasion resistance or fastening strength of the rigid internal gear.
To achieve the above and other objects, according to the present invention, there is provided a lightweight wave gear device which comprises a ring-shaped rigid internal gear, a ring-shaped flexible external gear, and a wave generator that flexes the external gear in a radial direction and has the flexible external gear partially engage the internal gear, while also moving engaging parts of the internal gear and the external gear in a circumferential direction, wherein the internal gear is made of one out of a group consisting of: aluminum alloy; copper alloy; titanium alloy; or magnesium alloy, and the internal gear is formed with internal teeth having a high profile.
A height of the internal teeth that are formed on the internal gear should normally be set at 1.1 to 1.3 times the height of conventional internal teeth that are formed on an internal gear made of a ferrous material.
To improve the abrasion resistance of the internal gear and suppress any falls in fastening strength, it is preferable for a plated coating of a material that has a higher modulus of elasticity than a material forming the internal gear to be formed on at least tooth surface parts, out of a surface of the internal gear.
The plated coating may be an electroless Nixe2x80x94P plated coating.
With the constitution of the present invention, the internal gear may be formed of one of an aluminum alloy with a modulus of elasticity that is at least 8500 kgf/mm2 and a titanium alloy.
In another aspect of the present invention, there is provided a lightweight wave gear device which comprises a ring-shaped rigid internal gear, a ring-shaped flexible external gear, and a wave generator that flexes the external gear in a radial direction and has the flexible external gear partially engage the internal gear, while also moving engaging parts of the internal gear and the external gear in a circumferential direction, wherein the internal gear is formed of one of an aluminum alloy with a hardness of at least Hv130 and a titanium alloy, and an electrolessly plated coating is formed on at least tooth surface parts out of a surface of the internal gear.
Here, it is preferable for the internal gear to have a hardness of at least Hv150 and a tensile strength of 40 kgf/mm2.
It is preferable for there to be little difference in hardness between the surface of the main material of the internal gear and the plated coating. Accordingly, it is preferable for the parts of the surface of the internal gear on which the electrolessly plated coating is formed to be subjected to a surface hardening process achieved through shot peening.
The electrolessly plated coating may be made of one of Nixe2x80x94P, Nixe2x80x94Pxe2x80x94B, and Cr.
The thickness of the electrolessly plated coating may be set in a range of 5 to 40 microns.
The hardness of the electrolessly plated coating needs to correspond to the hardness of the flexible external gear that the rigid internal gear engages. Normally, the hardness of the electrolessly plated coating may be set in a range of Hv300 to Hv1200.
In order to suppress falls in the fastening strength with which the rigid internal gear is fastened to other members, it is preferable for the electrolessly plated coating to be formed on ring-shaped end surfaces on both sides of the internal gear so as to increase the hardness of the surface parts that form the bearing surfaces for fastening bolts.